Body Fluids and Acid-Base Balance

Acid-Base Balance

pH, Buffers, Acids, and Bases

Acids dissociate into H+ and lower pH, while bases dissociate into OH− and raise pH; buffers can absorb these excess ions to maintain pH.

Learning Objectives

Explain the composition of buffer solutions and how they maintain a steady pH

Key Takeaways

Key Points

A basic solution will have a pH above 7.0, while an acidic solution will have a pH below 7.0.

Buffers are solutions that contain a weak acid and its a conjugate base; as such, they can absorb excess H+ ions or OH– ions, thereby maintaining an overall steady pH in the solution.

pH is equal to the negative logarithm of the concentration of H+ ions in solution: pH = −log[H+].

Key Terms

alkaline: having a pH greater than 7; basic

acidic: having a pH less than 7

buffer: a solution composed of a weak acid and its conjugate base that can be used to stabilize the pH of a solution

Self-Ionization of Water

Hydrogen ions are spontaneously generated in pure water by the dissociation (ionization) of a small percentage of water molecules into equal numbers of hydrogen (H+) ions and hydroxide (OH−) ions. The hydroxide ions remain in solution because of their hydrogen bonds with other water molecules; the hydrogen ions, consisting of naked protons, are immediately attracted to un-ionized water molecules and form hydronium ions (H30+). By convention, scientists refer to hydrogen ions and their concentration as if they were free in this state in liquid water.

The concentration of hydrogen ions dissociating from pure water is 1 × 10−7 moles H+ ions per liter of water. The pH is calculated as the negative of the base 10 logarithm of this concentration:

pH = −log[H+]

The negative log of 1 × 10−7 is equal to 7.0, which is also known as neutral pH. Human cells and blood each maintain near-neutral pH.

pH Scale

The pH of a solution indicates its acidity or basicity (alkalinity). The pH scale is an inverse logarithm that ranges from 0 to 14: anything below 7.0 (ranging from 0.0 to 6.9) is acidic, and anything above 7.0 (from 7.1 to 14.0) is basic (or alkaline ). Extremes in pH in either direction from 7.0 are usually considered inhospitable to life. The pH in cells (6.8) and the blood (7.4) are both very close to neutral, whereas the environment in the stomach is highly acidic, with a pH of 1 to 2.

The pH scale: The pH scale measures the concentration of hydrogen ions (H+) in a solution.

Non-neutral pH readings result from dissolving acids or bases in water. Using the negative logarithm to generate positive integers, high concentrations of hydrogen ions yield a low pH, and low concentrations a high pH.

An acid is a substance that increases the concentration of hydrogen ions (H+) in a solution, usually by dissociating one of its hydrogen atoms. A base provides either hydroxide ions (OH−) or other negatively-charged ions that react with hydrogen ions in solution, thereby reducing the concentration of H+ and raising the pH.

Strong Acids and Strong Bases

The stronger the acid, the more readily it donates H+. For example, hydrochloric acid (HCl) is highly acidic and completely dissociates into hydrogen and chloride ions, whereas the acids in tomato juice or vinegar do not completely dissociate and are considered weak acids; conversely, strong bases readily donate OH− and/or react with hydrogen ions. Sodium hydroxide (NaOH) and many household cleaners are highly basic and give up OH− rapidly when placed in water; the OH− ions react with H+ in solution, creating new water molecules and lowering the amount of free H+ in the system, thereby raising the overall pH. An example of a weak basic solution is seawater, which has a pH near 8.0, close enough to neutral that well-adapted marine organisms thrive in this alkaline environment.

Buffers

How can organisms whose bodies require a near-neutral pH ingest acidic and basic substances (a human drinking orange juice, for example) and survive? Buffers are the key. Buffers usually consist of a weak acid and its conjugate base; this enables them to readily absorb excess H+ or OH–, keeping the system’s pH within a narrow range.

Maintaining a constant blood pH is critical to a person’s well-being. The buffer that maintains the pH of human blood involves carbonic acid (H2CO3), bicarbonate ion (HCO3–), and carbon dioxide (CO2). When bicarbonate ions combine with free hydrogen ions and become carbonic acid, hydrogen ions are removed, moderating pH changes. Similarly, excess carbonic acid can be converted into carbon dioxide gas and exhaled through the lungs; this prevents too many free hydrogen ions from building up in the blood and dangerously reducing its pH; likewise, if too much OH– is introduced into the system, carbonic acid will combine with it to create bicarbonate, lowering the pH. Without this buffer system, the body’s pH would fluctuate enough to jeopardize survival.

Buffers in the body: This diagram shows the body’s buffering of blood pH levels: the blue arrows show the process of raising pH as more CO2 is made; the purple arrows indicate the reverse process, lowering pH as more bicarbonate is created.

Antacids, which combat excess stomach acid, are another example of buffers. Many over-the-counter medications work similarly to blood buffers, often with at least one ion (usually carbonate) capable of absorbing hydrogen and moderating pH, bringing relief to those that suffer “heartburn” from stomach acid after eating.

Chemical Buffer Systems

Chemical buffers, such as bicarbonate and ammonia, help keep the blood’s pH in the narrow range that is compatible with life.

Learning Objectives

Distinguish between buffer solutions, ventilation, and renal function as buffer systems to control acid–base balance

Key Takeaways

Key Points

The body’s acid– base balance is tightly regulated to keep the arterial blood pH between 7.38 and 7.42. Buffer solutions keep the pH constant in a wide variety of chemical actions.

A buffer solution is a mixture of a weak acid and its conjugate base, or a weak base and its conjugate acid.

Key Terms

bicarbonate: An alkaline, vital component of the pH buffering system of the human body that maintains acid–base homeostasis.

buffer: A solution used to stabilize the pH (acidity) of a liquid.

pH: In chemistry, a measure of the activity of the hydrogen ion concentration.

Examples

Anything that adversely affects an individual’s bloodstream will have a negative impact on that individual’s health since the blood acts as a chemical buffer solution to keep all the body’s cells and tissues properly balanced.

Acid–Base Homeostasis

Acid–base homeostasis concerns the proper balance between acids and bases; it is also called body pH. The body is very sensitive to its pH level, so strong mechanisms exist to maintain it. Outside an acceptable range of pH, proteins are denatured and digested, enzymes lose their ability to function, and death may occur.

Buffer Solution

A buffer solution is an aqueous solution of a weak acid and its conjugate base, or a weak base and its conjugate acid. Its pH changes very little when a small amount of strong acid or base is added to it. Buffer solutions are used as a means of keeping pH at a nearly constant value in a wide variety of chemical applications.

Many life forms thrive only in a relatively small pH range, so they utilize a buffer solution to maintain a constant pH. One example of a buffer solution found in nature is blood. The body’s acid–base balance is normally tightly regulated, keeping the arterial blood pH between 7.38 and 7.42.

Several buffering agents that reversibly bind hydrogen ions and impede any change in pH exist. Extracellular buffers include bicarbonate and ammonia, whereas proteins and phosphates act as intracellular buffers.

The bicarbonate buffering system is especially key, as carbon dioxide (CO2) can be shifted through carbonic acid (H2CO3) to hydrogen ions and bicarbonate (HCO3−):

Acid–base imbalances that overcome the buffer system can be compensated in the short term by changing the rate of ventilation. This alters the concentration of carbon dioxide in the blood and shifts the above reaction according to Le Chatelier’s principle, which in turn alters the pH.

Renal Physiology

The kidneys are slower to compensate, but renal physiology has several powerful mechanisms to control pH by the excretion of excess acid or base. In response to acidosis, the tubular cells reabsorb more bicarbonate from the tubular fluid, and the collecting duct cells secrete more hydrogen and generate more bicarbonate, and ammoniagenesis leads to an increase of the NH3 buffer.

In its responses to alkalosis, the kidneys may excrete more bicarbonate by decreasing hydrogen ion secretion from the tubular epithelial cells, and lower the rates of glutamine metabolism and ammonium excretion.

pH range: Buffering agents keep blood pH between 7.38 and 7.42.

Regulation of H+ by the Lungs

Acid–base imbalances in the blood’s pH can be altered by changes in breathing to expel more CO2 and raise pH back to normal.

Learning Objectives

Describe the regulation of hydrogen ions by the lungs

Key Takeaways

Key Points

Hydrogen ions (H+) are carried in the blood along with oxygen and carbon dioxide.

Sixty percent of the carbon dioxide is carried as dissolved bicarbonate.

A small amount of carbon dioxide is carried on the hemoglobin as carbaminohemoglobin, which is transported to the lungs for removal.

Following Le Chatelier’s principle, an imbalance in pH is returned to normal by increasing the rate of ventilation in the lungs.

To compensate for acidemia, more CO2 is expelled, while the opposite occurs for alkalemia.

Key Terms

carbaminohemoglobin: A compound of hemoglobin and carbon dioxide. It is one of the forms in which carbon dioxide exists in the blood.

Le Chatelier’s principle: A principle that states that if a chemical system at equilibrium experiences a change in concentration, temperature, or total pressure, the equilibrium will shift in order to minimize that change.

Examples

Since maintaining normal pH is vital for life, and since the lungs play a critical role in maintaining normal pH, smokers have yet another reason to quit smoking.

Acid–base imbalance occurs when a significant insult causes the blood pH to shift out of its normal range (7.35 to 7.45). An excess of acid in the blood is called acidemia and an excess of base is called alkalemia.

The process that causes the imbalance is classified based on the etiology of the disturbance (respiratory or metabolic) and the direction of change in pH ( acidosis or alkalosis). There are four basic processes and one or a combination may occur at any given time.

Metabolic acidosis

Respiratory acidosis

Metabolic alkalosis

Respiratory alkalosis

Blood carries oxygen, carbon dioxide, and hydrogen ions (H+) between tissues and the lungs. The majority of CO2 transported in the blood is dissolved in plasma (60% is dissolved bicarbonate).

A smaller fraction is transported in the red blood cells that combine with the globin portion of hemoglobin as carbaminohemoglobin. This is the chemical portion of the red blood cell that aids in the transport of oxygen and nutrients around the body, but, this time, it is carbon dioxide that is transported back to the lung.

Acid–base imbalances that overcome the buffer system can be compensated in the short term by changing the rate of ventilation. This alters the concentration of carbon dioxide in the blood, shifting the above reaction according to Le Chatelier’s principle, which in turn alters the pH. The basic reaction governed by this principle is as follows:

When the blood pH drops too low (acidemia), the body compensates by increasing breathing to expel more CO2; this shifts the above reaction to the left such that less hydrogen ions are free; thus, the pH will rise back to normal. For alkalemia, the opposite occurs.

The Role of the Kidneys in Acid-Base Balance

The kidneys help maintain the acid–base balance by excreting hydrogen ions into the urine and reabsorbing bicarbonate from the urine.

Learning Objectives

Describe the role of the kidneys to maintain the acid–base balance

Key Takeaways

Key Points

The kidneys maintain homeostasis through the excretion of waste products.

Acidosis causes more bicarbonate to be reabsorbed from the tubular fluid, while the collecting ducts secrete more hydrogen to generate more bicarbonate, and more NH3 buffer is formed.

Alkalosis causes the kidney to excrete more bicarbonate as there is a reduced secretion of hydrogen ions and more ammonium is excreted.

Key Terms

base: Any of a class of generally water-soluble compounds, that have a bitter taste, turn red litmus paper blue, and react with acids to form salts.

renal: Pertaining to the kidneys.

Examples

Urine testing is important because it can detect acid–base imbalances. For instance, uncontrolled diabetes results in highly acidic urine. If the diabetes remains uncontrolled, the kidneys could become over-stressed and malfunction, which could lead to coma or death.

Within the human body, fluids such as blood must be maintained within the narrow range of 7.35 to 7.45, making it slightly alkaline. Outside that range, pH becomes incompatible with life; proteins are denatured and digested, enzymes lose their ability to function, and the body is unable to sustain itself.

To maintain this narrow range of pH the body has a powerful buffering system. Acid–base imbalances that overcome this system are compensated in the short term by changing the rate of ventilation.

Kidneys and Acid–Base Balance

The kidneys have two very important roles in maintaining the acid–base balance:

They reabsorb bicarbonate from urine.

They excrete hydrogen ions into urine.

The kidneys are slower to compensate than the lungs, but renal physiology has several powerful mechanisms to control pH by the excretion of excess acid or base. The major, homeostatic control point for maintaining a stable pH balance is renal excretion.

Bicarbonate (HCO3−) does not have a transporter, so its reabsorption involves a series of reactions in the tubule lumen and tubular epithelium. In response to acidosis, the tubular cells reabsorb more bicarbonate from the tubular fluid, and the collecting duct cells secrete more hydrogen and generate more bicarbonate, and ammoniagenesis leads to an increase in the formation of the NH3 buffer.

In response to alkalosis, the kidneys may excrete more bicarbonate by decreasing hydrogen ion secretion from the tubular epithelial cells, and lowering the rates of glutamine metabolism and ammonium excretion.